X-ray tube target

ABSTRACT

A target for anodes of X-ray tubes, particularly rotating anodes, having a compensated bimetallic structure which comprises a base matrix carrying coatings of selected material on both opposed surfaces for resisting distortion caused by high thermal energies.

Baum

States atet r1 1 1 X-RAY TUBE TARGET [75] Inventor: Charles L. Baum,Stamford, Conn.

[73] Assignee: The Machlett Laboratories,

Incorporated, Springdale, Conn.

[22] Filed: Feb. 27, 1973 [21] Appl. No.: 336,283

[52] 11.8. C1 313/60, 313/330 [51] Int. Cl. 1101i 35/10 [58] Field ofSearch 313/60, 330

[56] References Cited UNITED STATES PATENTS 3,622,824 11/1971 Atlee313/60 3,710,170 1/1973 Friedel 313/60 Feb. 5, 1974 3,731,128 5/1973Haberrecker 313/60 FORElGN PATENTS OR APPLICATIONS 1,913,793 10/1970Gennany 313/60 Primary Examiner-Herman Karl Saalbach AssistantExaminer1Darwin R. Hostetter Attorney, Agent, 'or Firm-Harold A. Murphy;Joseph D. Pannone; John T. Meaney [57] STRACT A target for anodes ofX-ray tubes, particularly rotating anodes, having a compensatedbimetallic structure which comprises a base matrix carrying coatings ofselected material on both opposed surfaces for resisting distortioncaused by high thermal energies.

, 4 Drawing Figures X-RAY TUBE TARGET BACKGROUND OF THE INVENTION InX-ray tubes, aprincipal problem relates to the dissipation from theanode of heat which is produced by electron bombardment thereof. This isparticularly true of rotating anodes where cooling takes place princi-Jpally by radiation.

Tungsten has been well known as a material which efficiently producesX-ray when bombarded by electrons. However, when anodes are made ofso-called :puretungsten, at high thermal energies radial crackinfg andcrystalline separation or 'flaking commonly occu rgprobably becausesurface expansion occurs more quickly andis much greater than that ofthe interior of the target body.

Other 'high atomic number materials such as, for example, molybdenum and"graphite are also well suited for use as target materials because oftheir relatively light weight and fairly good thermal capacities. Suchtargets in the past have generally been coated in the area of the focalspot with alayer of tungsten or combination of tungsten andrhenium. Thishas resulted in some reduction in cracking and flaking. However,undesirable characteristics still occur to a very undesirable extent andare commonly evidenced as warping or distortion of the layer, inaddition to cracking.

The heat developed by an anode will vary considerably. For example, a0.3mm square projected focal spot on a target when bombarded withelectrons for 0.01 second will develop heat of as much as 8-12 kw. A 2mm square projected focal spot on a 10 target, bombarded for 0.01second, will develop more than 100 kw of heat. Such heat will cause thetarget to distort or bend in adirection toward the electron source.Since the focal-area of the target is inclined at a selected angle, suchas 10, such bending will produce a resultant decrease in the size of theprojected focal spot, sometimes to the point where the projected focalspot becomes nonexistent, resulting in an X-ray tube which will not emitany useful X-radiation.

Many attempts have been made to reduce the heat problem by increase ofactual surface area or by coating the surface of the'anode with amaterial having high thermalemissivity. Schram, in U. 5. Pat. No.

2,863,083, for example, teaches a molybdenum, graphite or boron targetcoated on its focal surface with a layer of tungsten and thenadditionally coated with a layer of rhenium which may extend over justthe focal surface or over both the sides and edges of the target. Schramattempts to create high thermal emissivity.

British specification No. 616,490 teaches the use of a molybdenum dischaving a thin tungsten layer in its bombarded surface. Ochsner et al inU. S. Pat. No. 3,1 l2,l 85 discloses an anode for electron dischargedevices, which anode comprises a five-layer structure which is intendedto prevent thermal deflection characteristics by virtue of the fact thatthe composite structure is an assymmetrical body comprised of materialhaving different thermal expansion, such materials being copper coatedon opposite surfaces with steel overlayed with aluminum.

Neither Schram nor the British invention has proved to be'successful,while the Ochsner et al. materials cannot be used in the fabrication ofrotating X-ray targets for many reasons. i

SUMMARY OF THE INVENTION In accordance with the objectives of thisinvention, there is provided an X-ray tube target which is soconstructed as to eliminate warping, bending or distortion prevalent intargets having high thermal emissivity coatings only on the focal sideof the target. This is achieved by the provision of a compensatedbimetallic structure which embodies a deposit or cladding of the highthermal emissivity coating material on both the focal side and theopposite side of the target, which coating is deposited directly uponthe base substrate material to a thickness greater than 0.010 inch,since thinner coatings will provide a structure wherein the temperaturedifferential between the substrate and the coating is too small, leadingto substrate melting beneath the focal spot. Coatings up to about 0.050inch are satisfactory, coatings of greater thickness being fabricatablebut impractical due to unnecessary use of expensive materials and thefact that the coating will be so thick as to nullify the benefit of thelight weight, high heat capacity substrate.

The base substrate. conveniently may be molybdenum coated on bothsurfaces with tungsten or tungsten rhenium alloy. The substrate-coatingthickness ratio will preferably be about 4-7 to 1.

Additionally, a fourth layer of machinable material is disposed upon thesurface coating which is disposed on the side of the target away fromthe electron source. Such an additional layer has been found to preventthe weakening of the adjacent rigidizing layer by loss of strengthinduced by mechanically treating the layer during target balancingprocedures.

Since the greatest concentration of heat is developed in the marginal orfocal area of the target, the present invention is directed particularlyto the provision of means in this area for preventing distortion byheat. Therefore, in one embodiment of the invention a ring of rigidizingmaterial is embedded within the base material of the target in the focalarea thereof.

In another embodiment, the layer of rigidizing material is made to besubstantially thicker opposite the focal area than the remainderthereof.

With a structure as set forth herein, the compensated bimetallic featureresults in considerably less warpage, compared to prior art types oftarget structures, and permits X-ray tubes to be operated at higherthermal loadings without damage.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevational view of anX-ray tube having a rotating target embodying the invention;

FIG. 2 is a sectional view through a target embodying this invention;and

FIGS. 3 and 4 are fragmentary sectional views of a target embodying twofurther embodiments of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring more particularly tothe drawing, there is shown in FIG. 1, by way of example, one type ofX-ray generator tube having a rotatable anode. The glass envelope 10,which contains a high vacuum, houses the cathode 121 which is mounted ona suitablecathode support structure l4l sealed into one end of theenvelope. A rotor 16 is sealed into the opposite end of the envelope andhas extending inwardly from it an operating shaft 18 on the inner end ofwhich is a transversely extending disc-like anode target 20. The cathode12 is offset with respect to the center of rotation of shaft 18 andtarget 20 so as to direct electrons onto an inclined annular surface 22of the target. In the normal and well-known manner, the electrons fromcathode 12 are focussed onto a small area of the target, this area beingof predetermined size, such as 0.3 mm square projected, 2 mm squareprojected, or other square, rectangular or other configuration and sizeas desired.

The anode and cathode structures are connected to suitable sources ofpotential, as is conventional, to produce a stream of electrons whichimpinge upon the focal spot for a predetermined pulse duration such as0.01 second, for example. During this process the anode target 20 isrotated at a high speed. Thus, the focal spot traverses an annular trackon the inclined surface 22 during the pulse duration. Impingement ofelectrons on the target surface creates considerable heat not only onthe surface of the target but within the body thereof, as is well known,particularly in the portion opposite the focal track.

The target 20 comprises a circular disc 24 of molybdenum or graphite(FIG. 2) which is critically balanced to rotate about its geometriccenter, and is coated on its surface adjacent the cathode 12 with alayer 26 of tungsten or tungsten-rhenium alloy. However, theparticularly important feature of this invention resides in the factthat a compensated bimetallic structure is achieved by also providing alayer 28 of the same tungsten or tungsten-rhenium alloy or otherrigidizing material on the back surface of the target; that is, thesurface opposite the surface which is subjected to electron bombardment.

Layers 26 and 28 must be at least 0.010 inch thick but, forpracticality, should not exceed 0.050 inch maximum thickness. Theselayers may be pure tungsten or may comprise an alloy comprising tungstenor rhenium, in which case the tungsten may comprise about 90 percent andrhenium about percent. The selected coating material or materials areground, mixed, pressed and sintered to the molybdenum surfaces byconventional, well-known processes.

It has been found that when a molybdenum base 24 is provided with only asingle layer 26, the high thermal stresses to which the target issubjected during processing and operation of an X-ray tube will causeconsiderable warping and rippling of the layer 26, particularly in thefocal track area. The molybdenum base material deforms at hightemperature, but resists complete reverse deformation upon cooling,resulting in a lessening of the target angle, that is, the angle ofinclination of inclined surface 22.

It is known that molybdenum has an expansion coefficient of about 49 X10 inch per inch per degree Centigrade. Tungsten expands only about 26 X10 inch per inch per degree Centigrade. Therefore, it will be apparentthat when the anode target is heated, the molybdenum disc 24 will expandand, in doing so, will tend to flatten from its slightly cup-shapedconfiguration. This will not only alter the angle of the inclinedsurface 22, that is, alter the actual spacing between surface 22 and thecathode 12, resulting in change in focal spot size, but will alsoproduce slight distortions such as convolutions or ripples in thecoating 26.

In accordance with this invention the compensated bimetallic effectcreated by the second layer 28 produces a resistance to such deformationof the molybdenum disc 24 and consequently also reduces the tendency ofthe layer 26 to warp or ripple. Cracks and crazing or flaking are alsoless common, resulting in more efficient X-ray generation.

It is, of course, necessary that the rigidizing layer 28 be a materialwhich has a substantially lower thermal expansion coefficient than thebase material 24, but it should also be a material which may besatisfactorily used in X-ray tubes without substantially increasing theweight of the target.

In FIG. 2, the layer 28 is shown to be a layer of substantially uniformthickness extending across the entire lower surface of the disc 24.

In FIG. 3, the layer 28a is shown to be substantially thicker in theregion of the focal area or the marginal area of the target, than in theinterior portions thereof. For this purpose the layer is made to taperand thicken progressively outwardly from the center.

In FIG. 4 the rigidizing layer 28b is made in the form ofa ring which isimbedded within the material of disc 24 at a level between the twoopposed surfaces of the inclined focal area. Thus, the rigidization isconfined to the area of the target where the greatest buildup of heatoccurs.

In further accordance with this invention, there is provided anadditional layer 30 on the back or bottom surface of the target as shownin FIGS. 2 and 3. Layer 30 overlies layer 28 and is preferably depositedor formed as a relatively uniformly thick layer. Layer 30 is of a metalhaving easy mechanical workability and may be molybdenum or graphite,for example, similar to the material of the base disc 24. It has beenfound that when the rigidizing layer 28 was abraded or otherwisemechanically treated during the balancing of the target, it sometimesbecame weakened sufficiently to lose its strengthening ordistortion-preventing affect upon the disc. The added layer 30 thereforeallows the rigidizing layer 28 to perform adequately while providing initself a means whereby the target may be balanced.

From the foregoing it will be apparent that all of the objectives ofthis invention have been achieved by the structure shown and described.Other objectives and advantages of the invention will become apparent tothose skilled in the art. Therefore, the structures shown and describedare to be interpreted as illustrative.

I claim:

1. An X-ray tube having an electron-emitting cathode and an anodepositioned in the path of electrons from the cathode, said anodecomprising a rotatable shaft, and a disclike target on the shaft androtatable therewith, the target being disclike in shape and having aselected portion disposed to rotate through the path of the electrons,said target comprising a substrate having a known coefficient of thermalexpansion and having on both opposed surfaces layers of material whichhas a higher coefficient of thermal expansion than the material of thesubstrate, and a metallic coating overlying the layer on the side of thetarget remote from the cathode.

2. An X-ray tube as set forth in claim 1 wherein said substrate ismolybdenum and said layers are a metal selected from the groupconsisting of tungsten and tungsten-rhenium alloy.

3. An X-ray tube as set forth in claim 2 wherein said metallic coatingis molybdenum.

4. An X-ray tube as set forth in claim 2 wherein said metallic coatingis graphite.

5. An X-ray tube as set forth in claim 1 wherein said substrate isgraphite and said layers are a metal selected from the group consistingof tungsten and tungstenrhenium alloy.

6. An X-ray tube as set forth in claim 4 wherein said metallic coatingis graphite.

7. An X-ray tube as set forth in claim 4 wherein said metallic coatingis molybdenum.

8. An X-ray tube as set forth in claim 1 wherein the layer on thesurface of the target facing the cathode is confined substantially tosaid selected portion thereof.

both sides by material of the substrate.

1. An X-ray tube having an electron-emitting cathode and an anodepositioned in the path of electrons from the cathode, said anodecomprising a rotatable shaft, and a disclike target on the shaft androtatable therewith, the target being disclike in shape and having aselected portion disposed to rotate through the path of the electrons,said target comprising a substrate having a known coefficient of thermalexpansion and having on both opposed surfaces layers of material whichhas a higher coefficient of thermal expansion than the material of thesubstrate, and a metallic coating overlying the layer on the side of thetarget remote from the cathode.
 2. An X-ray tube as set forth in claim 1wherein said substrate is molybdenum and said layers are a metalselected from the group consisting of tungsten and tungsten-rheniumalloy.
 3. An X-ray tube as set forth in claim 2 wherein said metalliccoating is molybdenum.
 4. An X-ray tube as set forth in claim 2 whereinsaid metallic coating is graphite.
 5. An X-ray tube as set forth inclaim 1 wherein said substrate is graphite and said layers are a metalselected from the group consisting of tungsten and tungsten-rheniumalloy.
 6. An X-ray tube as set forth in claim 4 wherein said metalliccoating is graphite.
 7. An X-ray tube as set forth in claim 4 whereinsaid metallic coating is molybdenum.
 8. An X-ray tube as set forth inclaim 1 wherein the layer on the surface of the target facing thecathode is confined substantially to said selected portion thereof. 9.An X-ray tube as set forth in claim 1 wherein the layer on the surfaceof the target remote from the cathode varies in thickness in the radialdirection.
 10. An X-ray tube as set forth in claim 9 wherein the layeron the surface of the target remote from the cathode is substantiallythicker in the area thereof opposite said selected portion of thetarget.
 11. An X-ray tube as set forth in claim 1 wherein the layer moreremote from the cathode is enclosed on both sides by material of thesubstrate.